Numerous systems are known for controlled and programmed release of active principals.
They are all solid or semi-solid systems, e.g. of the tablet or suppository type having a composition which is defined so as to lead to the systems breaking up slowly, with breaking up itself being obtained either by a purely mechanical effect, or else by partial dissolution of one or more components forming the system in a liquid medium, e.g. constituted by body fluids.
It is also known to disperse the active principal throughout a solid substrate and to obtain programmed release thereof by diffusion through such a substrate.
In this case also, numerous systems are described:
there are "semipermeable" systems in which the active agent dissolved, suspended, or emulsified in an inert vehicle diffuses through a permeable membrane; and
there are also compound systems in which the active agent diffuses from a more or less porous solid substrate constituted by a matrix based on polymers or copolymers.
Regardless of the system chosen, release speeds are maintained only for a very short length of time, and the system is therefore not suitable for prolonged use.
Solid systems are also known that are suitable for application by contact, in particular on the skin.
Particular mention may be made of devices suitable for gradually releasing a contact insecticide for protecting animals against parasites, and in particular against ectoparasites. Most of these devices are constituted by an insecticide included in a matrix of plastic material. The slow release of an ectoparasiticide substance (whether in vapor form or by deposition on the hairs of the animal) provides effective protection against parasites for several months.
Such anti-parasite devices constitute the subject matter of very many patents, and in particular the Applicants' French patents numbers FR 2 447 679, FR 2 269 859, and FR 2 307 456, and also the following patents: FR 2 436 563 in the name of Shell International Research; FR 2 370 572 and FR 2 392 606 in the name of A. H. Robins Company Inc.; FR 1 598 644 and FR 2 213 014 in the name of Robert Aries; and FR 2 374 853, FR 2 386 253, and FR 2 386 254, in the name of Bayer AG, and such devices are essentially constituted by three basic components:
an insecticide (or mixture of insecticides);
a matrix (generally based on a thermoplastic resin); and
additives (such as plasticizers, pigments, stabilizing agents, or inert fills, etc. . . . . ).
Nevertheless, all prior art devices suffer from two major drawbacks:
they never release all of the active principal that they contain, and this may be important, or even essential, particularly with active principals that are expensive; and
the active principal is never released at a constant rate, with a very large quantity of substance being released at the beginning of utilization, after which the release rate falls off very rapidly to become zero.
These drawbacks of the prior art are related, in particular, to the active principal being applied hot and then being absorbed or adsorbed by the resin, which means that a large amount of the active principal is retained by the resin (of the order of 30% to 70%, depending on circumstances). This major retention of the active principal in prior art devices is due, in particular, to the following facts:
when the active principal is a liquid, it behaves like a plasticizer for the resin (two-way interchange between the plasticizer and the active principal), and in this case the active substance is absorbed by the resin; and
when the active principal is a solid, the plasticizer acts as a solvent therefor, such that the active principal is adsorbed by the plasticizer included in the resin.
Consequently, an object of the present invention is to provide a device which satisfies practical requirements better than previously known devices for the same purpose, and in particular to provide a device that releases all of the active principal contained in said device over varying, but predictable, durations, and above all making substantially constant daily release possible over the entire lifetime of the device.